Field of the Disclosure
This disclosure relates generally to implementing the asynchronous operation of permanent magnet motors (PMM), particularly though not exclusively in a downhole environment, such as a hydrocarbon (e.g., oil, gas, or mixed oil and gas) well.
Description of the Related Art
Electric submersible pump (ESP) systems are typically installed in oil and gas wells where reservoir pressure is inadequate to lift reservoir fluids to the surface or to increase production in natural producing wells. As a reservoir is produced, the pressure in the pore space of the rocks decreases, and thus may require the introduction of some type of artificial lift system to continue production as a reservoir or a well ages. An ESP system provides an artificial lift for a reservoir and/or well and comprises a motor to convert electrical power from a cable to mechanical power to drive the pump. ESP systems may be installed with induction motors (IMs) or PMMs.
Although ESP systems are used world-wide, ESP systems typically suffer from numerous problems that cause ESP systems to be relatively expensive to operate and susceptible to failure. For instance, when a typical ESP system fails, the entire production tubing string may be removed from the well to replace the ESP system. Removal of the entire production tubing string is relatively expensive and a time consuming process that may require a workover rig. In many geographic areas, workover rigs are not readily available or may be difficult to transport or deploy in offshore environments or other remote locations. In some instances, weeks or months may elapse in order to deploy a workover rig to replace a failed ESP system.
Moreover, many reservoirs are located within sandstone. As these reservoirs are produced, the sandstone often disintegrates, causing significant sand volumes to travel to the surface along with the produced fluid. The sand is normally removed at the surface and does not present a significant problem other than pump wear. However, when the sand becomes embedded in the pump stages, the sand can create very high loads in the ESP system by making the rotation of the pump difficult or nearly impossible, and thus may require replacement of the ESP system. Furthermore, difficultly in achieving or maintaining the desired rotation of the pump components can have adverse repercussions on the operating life of the pump motor, and in some instances can lead to damage or failure of the motor. For example, if the pump system is shut down during production, the sand in the fluid column can fall onto the ESP system. In other instances, changes in the reservoir and reservoir fluid can produce relatively large slugs of sand that congest and restrict rotation of the coupled pump and motor components of ESP system. Similarly, pipe scale, highly viscous fluids (particularly when operating the system in relatively cold environments), and other debris can create blockage of the pump, inhibit rotation of the coupled pump and motor components, and force removal and repair or replacement of the ESP system to continue production.